Reference for a preliminary ruling — Union citizenship — Article 20 TFEU — Access to social assistance and child benefit conditional on right of residence in a Member State — Third-country national responsible for the primary day-to-day care of her minor child, a national of that Member State — Obligation on the third-country national to establish that the other parent, a national of that Member State, is not capable of caring for the child — Refusal of residence possibly obliging the child to leave the territory of the Member State, or the territory of the European Union